Power transmissions networks can be made of AC systems, DC systems, or a combination of the two. AC power networks have conventionally been used throughout the world. However, DC power networks have certain advantages. DC power networks are easier to design and implement because they introduce no reactance into the power system. Higher efficiencies from generators can be achieved in DC systems because only real power is transmitted. Additionally, parallelization of power supplies is simple because the operating frequency of DC power supplies is 0 Hz. Therefore, no synchronization is required when additional supplies or loads are brought onto the network.
The conventional use of AC power networks is a result of the ease of transmitting AC power over long distances and handling voltage changes using transformers. However, over short distances, such as those in isolated environments, a DC power transmission network could be beneficial for the reasons previously explained. High-power generators available today typically produce AC power. Therefore, operation of a DC transmission network powered by AC generators requires conversion from AC to DC and vice versa.
Reliable operation of a power network is a critical element of many electronic systems, for example, on drilling platforms or vessels to operate onboard thrusters. Drilling vessels are not anchored in the ocean but are dynamically controlled to maintain a desired position in the ocean. Thrusters are used to maintain a position within specified tolerances of a drilling apparatus. Thrusters are propeller drives which can have variable rotation speed and azimuthal angle of the blades. These thrusters are operated by a power supply onboard the drilling vessel. Any failure of the power supply can lead to displacement of the vessel out of the tolerances of the drilling apparatus. In such a case, the drilling apparatus would need to be mechanically decoupled and recoupled after the power supply is restored and the position of the drilling vessel is corrected.
One method of facilitating a reliable power supply is to utilize a DC bus for powering thrusters and other components. Such a power transmission system is demonstrated in FIG. 1. In such a system, the power supply is generally made of AC generators coupled to an AC-to-DC converter. The AC-to-DC converter places power from the AC generators on an intermediate DC bus. The intermediate DC bus may be augmented with DC generators or a battery backup system. Each motor or thruster, as well as other devices utilizing the intermediate DC bus, on board the drilling vessel is coupled to the intermediate DC bus through a DC-to-AC converter.
FIG. 1 is a block diagram illustrating a conventional DC voltage bus coupling multiple AC voltage generation systems to various loads. Power system 100 includes generators 102. Generators 102 couple to AC bus 104 through isolators 106. Isolators 106 allow generators 102 to be removed from the bus when they are not needed or are malfunctioning. AC bus 104 couples to transformer 108 to condition the power for transmission to line 110. AC-to-DC converter 112 couples to line 110 and converts AC power to DC power for output onto intermediate DC bus 120. Coupled to DC bus 120 are DC-to-AC converters 130. DC-to-AC converters 130 convert DC power to AC power which most components are designed to use. Coupled to DC-to-AC converters 130 is line 132 to which loads may be connected. Motor 134 is coupled to line 132, and motor 134 could be, for example, a thruster. Additionally, transformer 135 is coupled to line 132 to condition power for load 136. Load 136 could be, for example, a light bulb.
There are several methods for implementing the AC-to-DC converter necessary for placing power from the AC generators on the intermediate DC bus. These methods conventionally employ the use of either diodes, silicon-controlled rectifiers (SCRs), or transistors.
One apparatus for AC-to-DC power conversion is a diode rectifier (or a diode pack). The are several forms of diode rectifiers commonly known. One typical diode rectifier is a full-wave diode rectifier. The AC power systems on drilling vessels typically utilize a three-phase waveform such that a six diode rectifier configuration is typically used. Diodes conduct current only when the voltage at the anode of the diode is greater than the voltage at the cathode of the diode. FIG. 2 is a schematic illustrating a conventional diode full-wave rectifier for three-phase AC power. Diode rectifier 200 accepts input from three-phase AC source 202. The rectifier 200 includes diodes 204 for rectifying the first phase, diodes 206 for rectifying the second phase, and diodes 208 for rectifying the third phase. Two diodes are needed in each case to produce output from both the positive AC cycle and the negative AC cycle. Diodes 204, diodes 206, and diodes 208 are coupled between the AC source 202 and the DC bus 210. Capacitor 212 is coupled to the DC bus 210 to average voltage ripples on DC bus 210. While rectifier 200 is shown as a single rectifier arrangement, several individual arrangements of one power capacity may be placed in parallel to create a rectifier 200 with a higher power capacity.
Diode rectifiers are commercially available from various vendors or can be constructed by arranging individually-purchased diodes. The advantages to diode rectifiers are the low cost of the components. Individual diodes and complete rectifiers are relatively inexpensive for high-power configurations, i.e., several megawatts (MW). Diodes are also relatively small devices compared to other available solutions at an equivalent power load. Diode rectifiers, however, have no ability to regulate the output voltage or current. Additionally, they only conduct in one direction.
As a result of the inability to regulate output voltage or current from diode rectifiers, SCRs, also known as thyristor rectifiers, have largely been used in their place. FIG. 3 is a schematic illustrating a conventional arrangement of SCRs for three-phase AC-to-DC conversion. SCR pack 300 accepts input from three-phase AC source 302. SCR pack 300 includes SCRs 304 for converting the first phase, SCRs 306 for converting the second phase, and SCRs 308 for converting the third phase. Each individual SCR includes a gate terminal 305 for accepting input. Two SCRs are needed in each case to produce output from both the positive AC cycle and the negative AC cycle. SCRs 304, SCRs 306, and SCRs 308 are coupled to AC source 302 and to DC bus 310. Capacitor 312 is coupled to the DC bus 310 to average ripples on DC bus 310. While SCR pack 300 is shown as a SCR arrangement, several individual arrangements of one power capacity may be placed in parallel to create a SCR pack 300 with a higher power capacity.
Output current may be regulated in the SCRs by controlling through gate terminal 305 when in the AC cycle they turn on. SCRs also offer the low cost, small size, and reliability of diodes. The disadvantage of SCRs is their slow switching time that must occur in synchronization with the AC power supplies. As a result, they are not well suited to handle the power load changes experienced during instability in the power system. Additionally, once the SCR is turned on through gate terminal 305, it may not be turned off through gate terminal 305.
Transistors offer yet another solution for AC-to-DC power conversion. FIG. 4 is a schematic illustrating a conventional arrangement of transistors for three-phase AC-to-DC power conversion. Transistor pack 400 accepts input from three-phase AC source 402. Transistor pack 400 includes transistors 404 to convert the first phase, transistors 406 to convert the second phase, and transistors 408 to convert the third phase. Additionally, diodes 405 are coupled on both sides to transistors 404 to protect transistors 404 from damaging voltages which may develop across transistors 404 and complete the power transfer circuit. This setup is repeated for diodes 407 coupled to transistors 406 and diodes 409 coupled to transistors 408. Inductors 403 condition the power before reaching transistors 404, transistors 406, and transistors 408. Transistors 404, transistors 406, transistors 408 are coupled to AC source 402 and to DC bus 410. Capacitor 412 is coupled to the DC bus 410 to average ripples on DC bus 410. While transistor pack 400 is shown as a transistor arrangement, several individual arrangements of one power capacity may be placed in parallel to create a transistor pack 400 with a higher power capacity.
Transistors possess faster switching characteristics than SCRs as well as the ability to control on and off timing, making them a better solution under transients resulting from real loads. Additionally, transistors allow power flow in both directions through the converter. This allows power to be moved back from the DC bus to the AC bus. It is typically required that multiple transistor-based conversion devices be placed in parallel to handle large loads. Transistors are expensive devices relative to diodes and SCRs and occupy significantly larger amounts of space. Additionally, transistors are fragile and break easily.
Thus, there is a need for a power system that has the fast switching capability of transistors and the low cost, durability, and small footprint of diodes or SCRs.